An X-ray CT measuring apparatus configured to obtain a tomographic image of a subject (measurement object) in a nondestructive manner has been known (see Japanese Patent Application Laid-Open Nos. 2002-55062 and 2004-12407 and Japanese Patent No. 5408873). The X-ray CT measuring apparatus performs X-ray irradiation while rotating the subject which is arranged at the center of a rotary table.
FIG. 1 shows a configuration of a typical X-ray CT apparatus used for measurement. An X-ray tube 12, an X-ray detector 14, a rotary table 16, and an XYZ moving mechanism unit 18 are accommodated in an enclosure 10 which shields X-rays. The X-ray tube 12 emits X-rays 13. The X-ray detector 14 detects the X-rays 13. A subject 8 is placed on the rotary table 16, and the rotary table 16 rotates the subject 8 for CT imaging. The XYZ moving mechanism unit 18 is intended to adjust the position and magnification of the subject 8 captured by the X-ray detector 14. The X-ray CT apparatus further includes a controller 20 which controls the devices, and a control personal computer (PC) 22 which gives instructions to the controller 20 on the basis of user operations.
Aside from the function of controlling the devices, the control PC 22 has a function of displaying a projection image of the subject 8 captured by the X-ray detector 14 and a function of reconstructing a tomographic image from a plurality of projection images of the subject 8.
When the X-rays 13 pass through an object, there occurs a considerable amount of scattered X-rays reflected in directions different from the irradiation direction. Such scattered X-rays are known to appear as noise in an X-ray CT imaging result. To suppress the scattered X-rays, an X-ray collimator 24 is arranged near the X-ray tube 12. To limit the irradiation range of the X-rays in a vertical direction, the X-ray collimator 24 includes parts, or an upper movable part 24A and a lower movable part 24B, that are made of a radiopaque material (such as tungsten). The parts 24A and 24B are each configured to be vertically movable. The positions of the upper and lower movable parts 24A and 24B of the X-ray collimator 24 are controlled by the control PC 22 according to the imaging range of the subject 8.
As shown in FIG. 2 (perspective view) and FIG. 3 (plan view), the X-rays 13 emitted from an X-ray source including the X-ray tube 12 are transmitted through the subject 8 on the rotary table 16 and reach the X-ray detector 14. The X-ray detector 14 obtains transmission images (projection images) of the subject 8 in all directions while rotating the subject 8. The transmission images are reconstructed by a reconstruction algorithm such as a back projection method or an iterative reconstruction method to generate a tomographic image of the subject 8.
The position of the subject 8 can be moved by controlling X-, Y-, and Z-axes of the XYZ moving mechanism unit 18 and a θ-axis of the rotary table 16. The imaging range (position and magnification) and the imaging angle of the subject 8 can be thereby adjusted.
Suppose, as shown in FIG. 4, that a center X0 of the rotary table 16 is moved toward the X-ray tube 12 along the X-axis. As shown in broken lines, the peripheral portion of the rotary table 16 can be moved up to a position X1 beyond the X-ray tube 12. The reason is that the X-ray tube 12 needs to be brought close to the subject 8 for enlarged imaging if the subject 8 is small. The X-ray tube 12 is structurally permitted to enter the range of the rotary table 16. The X-ray tube 12 and the rotary table 16 do not interfere with each other, since the X-ray tube 12 and the rotary table 16 are at different vertical positions as shown in FIG. 2.
A subject 8 that even falls inside the range of the rotary table 16 can thus interfere (collide) with the X-ray tube 12 if brought too close to the X-ray tube 12. Each time a subject 8 is set, an operator therefore needs to set a movement limit of the rotary table 16 in the X-axis direction in advance so that the X-ray tube 12 that emits the X-rays 13 does not interfere with the rotating subject 8 (see Japanese Patent Application Laid-Open Nos. 2004-45212 (paragraphs 0305 to 0308), 2004-301860 (paragraphs 0286 to 0289), and 2004-301861 (paragraphs 0286 to 0289)).
Specifically, an interference prevention procedure is performed as follows:
(1) Rotate the subject 8 on the rotary table 16 at position X0, and move the subject 8 by visual estimation to a direction (θ1) in which the subject 8 can interfere with the X-ray tube 12 during rotation.
(2) Bring the rotary table 16 close to the X-ray tube 12 while taking care not to cause interference between the X-ray tube 12 and the subject 8.
(3) At a position (X1) where the X-ray tube 12 and the subject 8 are sufficiently close to each other, rotate the subject 8 again while paying attention so that the subject 8 does not interfere with the X-ray tube 12. If interference is likely to occur, move the rotary table 16 somewhat away from the X-ray tube 12.
(4) If it is confirmed that the subject 8 does not cause interference during rotation, set the position (X1) as the movement limit value (lower limit) of the rotary table 16 in the X-axis direction.